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also the issue of accelerated wear of gearbox components by hydrogen
embrittlement. Hydrogen embrittlement is the process by which vari-
ous metals, including high-strength steel, become brittle and fracture
following exposure to hydrogen which is part of the water molecule.
Several different techniques are used by oil analysis laboratories
to determine the moisture content of lubricating oil but Karl Fisher
titration is the preferred method by wind turbine gearbox manufac-
turers and lubricant suppliers, as even small amounts (<100 ppm)
of water contamination can be detected in the oil using this method.
Through research performed by a reputable bearing manufacturer,
it was found that just 1 000 ppm of water contamination could reduce
ball bearing life by 70%. So in terms of condemning limits, best practice
suggests maintaining water levels at or below half of the saturation
level of the oil at its operating temperature. Thus, if the saturation level
is 1 000 ppm at 50°C, the caution level should be set at 500 ppm, with
the critical level at 1 000 ppm.
Oil cleanliness
Particle counting involves measuring the cleanliness of the oil and can
also be used to evaluate the effectiveness of lubricant filters.
Very much like water, particulate contamination is very damaging
to wind turbine gearboxes. It is for this reason that wind turbine man-
ufacturers have increasingly focused on oil cleanliness. Oil cleanliness
is critical to establishing equipment reliability, especially as there is a
direct correlation between oil cleanliness and component life.
In this technique the number of particles per millilitre of oil is
counted in a variety of size ranges starting at four microns and going
up to 100 microns. The total number of particles greater than four,
six and 14 microns are evaluated and assigned range numbers that
indicate the cleanliness of the oil.
It is particles of approximately the same size as the machine
clearances that have the greatest destructive potential. Particles the
size of or slightly larger than the oil film thickness enter the contact
zone and damage surfaces.
While this technique is effective in determining the number and
size of particles being generated, particle counting will not identify
what the particles are. They could be metallic – both ferrous and
non-ferrous, silica, silt, filter fibres, bacteria colonies, varnish agglom-
erations, water, etc.
The American Wind Energy Association and the American Gear
Manufacturers Association have released a technical standard that
sets attainable oil cleanliness targets [1].
Water contamination problems in
wind turbine gearboxes
Problem
Summary
Corrosion
Ionic currents in aqueous solution;
ptiing, leakage, breakage
Additive drop-out
Polar hydrophilic additives depletion, also
breaking colloidal suspensions of additive
particles; loss of additives, parts fouling
Microbial growth
Colonization of oils by bacteria and/or fungi;
acids, fouling slimes; health issue
Hydrolysis
Decomposition of ester-based fluids and
additives; loss of oil properties, acid and some-
times gel formation
Accelerated oil oxidation
Especially if metal wear debris present, rate
of oil oxidation increases by two orders of
magnitude; oil thickening, acidity
Surface-initiated Fatigue
Spalling
Water dissocates into O
2
and H
2
at tips of
propagating cracks. H
2
migrates into and weak-
ens stell by hydrogen embrittlement, cracks
spread faster, reducing life of rolling elements,
resulting in surface pits and craters
Source of sample
Iso Code
Oil added to gearbox
16/14/11
Gearbox after factory test
17/15/12
Gearbox after 24-72 hour service
17/15/12
Gearbox in service
18/16/13
Figure 8: Hydrogen embrittlement mechanism
(courtesy Noria Corporation).
Atomic Hydrogen
Diffusion
Void
H
2
Molecular
Hydrogen
H
+
H
+
H
+
+ e°
e°
e°
Wear Debris
Crack Networks
Hydrogen Embrittlement
After penetration, atomic hydrogen
reacts to form brittle compounds
and increases cracking.
Hydrogen Blistering
Concentration of hydrogen
in void increases, pressure
also increases cracking.
Figure 7: Water contamination problems in wind turbine gearboxes.
Figure 9: ANSI/AGMA/AWEA 6006-A01 Oil cleanliness
recommendations.
With rigorous particle contamination control, bearing life can increase
substantially resulting in greater gearbox reliability, uptime and en-
ergy production, extended warranty periods and a higher return on
investment.
Conclusion
Oil analysis provides a solid foundation on which to build an effective
condition monitoring programme in many applications. In the case
of wind turbine gearboxes, oil analysis has the potential to reduce
unscheduled maintenance, improve reliability and extend service life.
The oil analysis tests profiled in this article can help wind farm oper-
ators get maximum value from their oil sampling programme. When
these tests are performed on a routine basis and the results properly
analysed, oil analysis can facilitate the maintenance of wind turbine
gearboxes and, ultimately, support more widespread acceptance of
this promising form of power generation in South Africa.
Reference
[1] ANSI/AGMA/AWEA 6006-A01: Design and specification of gear-
boxes for wind turbines.
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ENERGY EFFICIENCY MADE SIMPLE 2015